1. Field of the Invention
The present invention relates to a welding method for high strength Cr-Mo steels, and particularly to a submerged arc welding method for high strength (2.25%-3%)Cr-1% Mo-steels containing Cr, Mo and V as essential components, and further Nb, Ti, B, Ca and the like as needed, to obtain weld metals excellent in strength at room temperature and high temperature, toughness, and creep strength, after stress relief (SR), temper brittleness resistance (less brittleness in service under high temperature environments), cold crack resistance (difficult-to-generate delayed fracture due to hydrogen) and SR crack resistance (difficult-to-generate intergranular crack due to age-hardening precipitation).
2. Description of the Related Art
Conventionally, (2.25%-3%)Cr-1% Mo-Steels have been extensively used for structures operated under high temperature/high pressure environments such as boilers and pressure vessels because of excellent high temperature characteristics. These structures being large in size and wall thicknesses have been mostly welded by a submerged arc welding method with a high welding efficiency. In recent years, to achieve the high efficient operation for the equipment, there is a tendency to enlarge the size and wall thickness of structures and to increase the temperature and pressure of the service environment. To meet the requirement, research activities have been made to develop high strength Cr-Mo steels added with V, Nb and the like.
Similarly, there have been strong demands in welding materials toward further improved performances in terms of high temperature strength, toughness, creep strength, temper brittleness resistance, cold crack resistance and SR crack resistance. To obtain weld metals having the above-described performances, various submerged arc welding methods have been disclosed in Unexamined Japanese Patent Publication Nos. SHO 61-71196, SHO 61-232089, SHO 63-238978, HEI 1-210193, HEI 1-271096, HEI 2-182378, HEI 3-258490, HEI 4-253595, and Examined Japanese Patent Publication No. HEI 4-79752. These methods, however, seem to be insufficient in the above-described performances of weld metals.